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heimdal/lib/asn1/README.md
Nicolas Williams 40d1271094 asn1: Expand decoration w/ C types 
This commits allows `heim_object_t` as a type and causes the generated
code to use the `heim_retain()` and `heim_release()` functions for
copying and releasing values of such types.

Also, now one can have more than one decoration per-type.
2022-01-11 10:21:05 -06:00

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# Heimdal's ASN.1 Compiler
This is a new README, and it's not very rich in contents yet. Be sure to check
out the [README on the template backend](/lib/asn1/README-template.md) and the [README
on automatic open type decoding via X.681/X.682/X.683
annotations](/lib/asn1/README-X681.md).
## Table of Contents
1. [Introduction](#Introduction)
2. [ASN.1 Support in Heimdal](#asn1-support-in-heimdal)
3. [News](#News)
4. [Features](#Features)
5. [Limitations](#Limitations)
6. [Compiler Usage](#Compiler-usage)
7. [APIs Generated by the Compiler](#APIs-generated-by-the-compiler)
8. [`asn1_print` Usage](#asn1_print-usage)
9. [Implementation](#implementation)
10. [Moving From C](#moving-from-c)
## Introduction
ASN.1 is a... some would say baroque, perhaps obsolete, archaic even, "syntax"
for expressing data type schemas, and also a set of "encoding rules" (ERs) that
specify many ways to encode values of those types for interchange.
Some ERs are binary, others are textual. Some binary ERs are tag-length-value
(TLV), others have no need for tagging. Some of the ERs are roundly and
rightly disliked, but then there are XER (XML Encoding Rules) and JER (JSON
Encoding Rules) that really illustrate how the syntax and the encoding rules
really are separate and distinct things.
ASN.1 is a wheel that everyone loves to reinvent, and often badly. It's worth
knowing a bit about it before reinventing this wheel badly yet again.
It's also worth pondering that there appears to be ways to map most data
exchange metaschemas and schemas onto others, and therefore too, transliterate
most encodings onto others.
First, an example of the syntax:
```ASN.1
-- This is what a certificate looks like (as in TLS server certificates, or
-- "SSL certs):
Certificate ::= SEQUENCE {
tbsCertificate TBSCertificate,
signatureAlgorithm AlgorithmIdentifier,
signatureValue BIT STRING
}
-- The main body of a certificate is here though:
TBSCertificate ::= SEQUENCE {
version [0] Version DEFAULT 1,
serialNumber CertificateSerialNumber,
signature AlgorithmIdentifier,
issuer Name,
validity Validity,
subject Name,
subjectPublicKeyInfo SubjectPublicKeyInfo,
issuerUniqueID [1] IMPLICIT BIT STRING OPTIONAL,
subjectUniqueID [2] IMPLICIT BIT STRING OPTIONAL,
extensions [3] EXPLICIT Extensions OPTIONAL
}
```
Here we see something akin to a "structure" or "record" with various named
fields of various types. Some of these are optional, which means they can have
no value given in encodings. One is defaulted, which means that if no values
is given in encodings then the default value is intended.
Those `[0]` things are called tags and are decidedly obsolete, along with all
"tag-length-value" (TLV) or "self-describing" encoding rules. Tags appear as
lexical tokens in ASN.1 only because a) in the early 80s TLV encodings were
thought fantastic, and b) automatic tagging wasn't invented and implemented
until it was too late. New ASN.1 modules should never need to have those tags
appear in the syntax.
ASN.1 has a lot of competition, and may even be obsolete. Obsolete
technologies take decades to die out because of the need to interoperate with
the installed base. So even if ASN.1 is obsolete, we find ourselves needing to
implement a large subset of it in order to implement certain important network
protocols.
Encoding rules? There are many:
- JSON Encoding Rules (JER) ([X.697](https://www.itu.int/rec/T-REC-X/recommendation.asp?lang=en&parent=T-REC-X.697))
Use JSON instead of some binary scheme like DER (see below).
- XML Encoding Rules (XER) ([X.693](https://www.itu.int/rec/T-REC-X/recommendation.asp?lang=en&parent=T-REC-X.693))
- Generic String Encoding Rules (GSER) ([RFC3641](https://tools.ietf.org/html/rfc3641))
- Basic, Distinguished, and Canonical Encoding Rules (BER, DER, CER) ([X.690](https://www.itu.int/rec/T-REC-X/recommendation.asp?lang=en&parent=T-REC-X.690)
These are the dreaded tag-length-value encoding rules. They are redundant,
wasteful, and inefficient in spite of being non-textual (i.e., binary)!
The descriptor "tag-length-value" is due to all values being encoded as some
bytes for a "tag", then some bytes for the length of the encoded value, then
the encoded value itself. The body of a structured type (e.g.,
`Certificate`) is itself a concatenation of the TLV encodings of the fields
of that structured type, in order.
DER and CER are alternative canonical forms of BER.
- Packed Encoding Rules (PER) ([X.691](https://www.itu.int/rec/T-REC-X/recommendation.asp?lang=en&parent=T-REC-X.691)) and Octet Encoding Rules (OER) ([X.696](https://www.itu.int/rec/T-REC-X/recommendation.asp?lang=en&parent=T-REC-X.696))
These are a lot like eXternal Data Representation
([XDR](https://tools.ietf.org/html/rfc4506.html)), but with 1-octet
alignment instead of 4-octet alignment.
There is also a meta encoding rule system, the Encoding Control Notation (ECN)
([X.692](https://www.itu.int/rec/T-REC-X/recommendation.asp?lang=en&parent=T-REC-X.692))
intended to be able to express all sorts of kinds of encodings.
Heimdal currently only supports DER for encoding, and DER and BER for decoding,
but soon may support JER as well, and can print values as JSON, though not
compliant with JER.
The syntax itself is specified by
[X.680](https://www.itu.int/rec/T-REC-X/recommendation.asp?lang=en&parent=T-REC-X.680),
with extensions via
[X.681](https://www.itu.int/rec/T-REC-X/recommendation.asp?lang=en&parent=T-REC-X.681),
[X.682](https://www.itu.int/rec/T-REC-X/recommendation.asp?lang=en&parent=T-REC-X.682),
and
[X.683](https://www.itu.int/rec/T-REC-X/recommendation.asp?lang=en&parent=T-REC-X.683),.
## ASN.1 Support in Heimdal
Heimdal contains an implementation of:
- ASN.1
- PKIX
- Kerberos
- misc. Heimdal-specific protocols related to PKIX and Kerberos, such as:
- Online certification authority protocols
- Kerberos KDC replication protocols
- Kerberos administration protocols
PKIX and Kerberos both require ASN.1 and DER support.
For historical reasons many ASN.1-using projects have used hand-rolled codecs
that have proven difficult to implement, maintain, and extend, and, of course,
buggy. Heimdal has its own ASN.1 module compiler and library in order to avoid
the pitfalls of hand-rolled codecs, and to satisfy Heimdal's internal needs.
There are other ASN.1 compilers and libraries out there, of course, but it
would prove difficult to switch compilers as generally ASN.1 compilers lack
sufficient control over generated types and APIs for programming languages.
Heimdal's ASN.1 compiler supports a large subset of X.680, X.681, X.682, and
X.683, as well as a large subset of X.690, with an architecture that should
make it easy to add support for encoding rules other than X.690.
## News
In recent times the following features have been added:
- Feature parity for the "template" backend, even superiority, as the codegen
backend does not yet support automatic open type decoding/encoding.
- IMPLICIT tagging support is finally complete.
- Automatic open type traversal, using a subset of X.681/X.682/X.683 for
expressing the requisite metadata.
- Decoration of ASN.1 types with "hidden" fields (ones that don't get encoded
or decoded) of ASN.1 or C types.
## Futures
- JER support?
- XDR/NDR/OER support?
- Generate comparators? (lib/hx509 has a half-baked Certificate comparator)
## Features
- Most of X.680 is supported.
- Most of X.690 is supported for decoding, with only DER supported for
encoding.
- Dumping of ASN.1 modules as JSON (note: the JSON schema for ASN.1 modules is
not stable yet). This enables analysis using, e.g., `jq` or similar, and
even construction of alternative compilers using `jq` or similar.
- We have an `asn1_print` program that can decode DER from any exported types
from any ASN.1 modules committed in Heimdal:
```bash
$ ./asn1_print ek.crt Certificate |
jq '.tbsCertificate.extensions[3]._extnValue[]._values'
```
```JSON
[
{
"_type": "TPMSpecification",
"family": "2.0",
"level": 0,
"revision": 138
}
]
[
{
"_type": "TPMSecurityAssertions",
"version": 0,
"fieldUpgradable": true,
"ekGenerationType": "ekgt-injected",
"ekGenerationLocation": "tpmManufacturer",
"ekCertificateGenerationLocation": "tpmManufacturer",
"ccInfo": {
"_type": "CommonCriteriaMeasures",
"version": "3.1",
"assurancelevel": "ealevel4",
"evaluationStatus": "evaluationCompleted",
"plus": true,
"strengthOfFunction": null,
"profileOid": null,
"profileUri": null,
"targetOid": null,
"targetUri": null
},
"fipsLevel": {
"_type": "FIPSLevel",
"version": "140-2",
"level": "sllevel2",
"plus": false
},
"iso9000Certified": false,
"iso9000Uri": null
}
]
```
A complete dump of such a certificate:
```bash
$ ./asn1_print ek.crt Certificate | jq .
```
```JSON
{
"_type": "Certificate",
"tbsCertificate": {
"_type": "TBSCertificate",
"_save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
"version": "rfc3280_version_3",
"serialNumber": "6A0597BA71D7E6D3AC0EDC9EDC95A15B998DE40A",
"signature": {
"_type": "AlgorithmIdentifier",
"algorithm": {
"_type": "OBJECT IDENTIFIER",
"oid": "1.2.840.113549.1.1.11",
"components": [
1,
2,
840,
113549,
1,
1,
11
],
"name": "id-pkcs1-sha256WithRSAEncryption"
},
"parameters": "0500"
},
"issuer": {
"_choice": "rdnSequence",
"value": [
[
{
"_type": "AttributeTypeAndValue",
"type": {
"_type": "OBJECT IDENTIFIER",
"oid": "2.5.4.6",
"components": [
2,
5,
4,
6
],
"name": "id-at-countryName"
},
"value": {
"_choice": "printableString",
"value": "CH"
}
}
],
[
{
"_type": "AttributeTypeAndValue",
"type": {
"_type": "OBJECT IDENTIFIER",
"oid": "2.5.4.10",
"components": [
2,
5,
4,
10
],
"name": "id-at-organizationName"
},
"value": {
"_choice": "printableString",
"value": "STMicroelectronics NV"
}
}
],
[
{
"_type": "AttributeTypeAndValue",
"type": {
"_type": "OBJECT IDENTIFIER",
"oid": "2.5.4.3",
"components": [
2,
5,
4,
3
],
"name": "id-at-commonName"
},
"value": {
"_choice": "printableString",
"value": "STM TPM EK Intermediate CA 05"
}
}
]
]
},
"validity": {
"_type": "Validity",
"notBefore": {
"_choice": "utcTime",
"value": "2018-12-14T00:00:00Z"
},
"notAfter": {
"_choice": "utcTime",
"value": "2028-12-14T00:00:00Z"
}
},
"subject": {
"_choice": "rdnSequence",
"value": []
},
"subjectPublicKeyInfo": {
"_type": "SubjectPublicKeyInfo",
"algorithm": {
"_type": "AlgorithmIdentifier",
"algorithm": {
"_type": "OBJECT IDENTIFIER",
"oid": "1.2.840.113549.1.1.1",
"components": [
1,
2,
840,
113549,
1,
1,
1
],
"name": "id-pkcs1-rsaEncryption"
},
"parameters": "0500"
},
"subjectPublicKey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
},
"issuerUniqueID": null,
"subjectUniqueID": null,
"extensions": [
{
"_type": "Extension",
"extnID": {
"_type": "OBJECT IDENTIFIER",
"oid": "2.5.29.35",
"components": [
2,
5,
29,
35
],
"name": "id-x509-ce-authorityKeyIdentifier"
},
"critical": false,
"extnValue": "301680141ADB994AB58BE57A0CC9B900E7851E1A43C08660",
"_extnValue_choice": "ext-AuthorityKeyIdentifier",
"_extnValue": {
"_type": "AuthorityKeyIdentifier",
"keyIdentifier": "1ADB994AB58BE57A0CC9B900E7851E1A43C08660",
"authorityCertIssuer": null,
"authorityCertSerialNumber": null
}
},
{
"_type": "Extension",
"extnID": {
"_type": "OBJECT IDENTIFIER",
"oid": "2.5.29.32",
"components": [
2,
5,
29,
32
],
"name": "id-x509-ce-certificatePolicies"
},
"critical": false,
"extnValue": "303930370604551D2000302F302D06082B060105050702011621687474703A2F2F7777772E73742E636F6D2F54504D2F7265706F7369746F72792F",
"_extnValue_choice": "ext-CertificatePolicies",
"_extnValue": [
{
"_type": "PolicyInformation",
"policyIdentifier": {
"_type": "OBJECT IDENTIFIER",
"oid": "2.5.29.32.0",
"components": [
2,
5,
29,
32,
0
],
"name": "id-x509-ce-certificatePolicies-anyPolicy"
},
"policyQualifiers": [
{
"_type": "PolicyQualifierInfo",
"policyQualifierId": {
"_type": "OBJECT IDENTIFIER",
"oid": "1.3.6.1.5.5.7.2.1",
"components": [
1,
3,
6,
1,
5,
5,
7,
2,
1
],
"name": "id-pkix-qt-cps"
},
"qualifier": "1621687474703A2F2F7777772E73742E636F6D2F54504D2F7265706F7369746F72792F"
}
]
}
]
},
{
"_type": "Extension",
"extnID": {
"_type": "OBJECT IDENTIFIER",
"oid": "2.5.29.17",
"components": [
2,
5,
29,
17
],
"name": "id-x509-ce-subjectAltName"
},
"critical": true,
"extnValue": "304DA44B304931163014060567810502010C0B69643A353335343444323031173015060567810502020C0C53543333485450484148433031163014060567810502030C0B69643A3030343930303038",
"_extnValue_choice": "ext-SubjectAltName",
"_extnValue": [
{
"_choice": "directoryName",
"value": {
"_choice": "rdnSequence",
"value": [
[
{
"_type": "AttributeTypeAndValue",
"type": {
"_type": "OBJECT IDENTIFIER",
"oid": "2.23.133.2.1",
"components": [
2,
23,
133,
2,
1
],
"name": "tcg-at-tpmManufacturer"
},
"value": {
"_choice": "utf8String",
"value": "id:53544D20"
}
}
],
[
{
"_type": "AttributeTypeAndValue",
"type": {
"_type": "OBJECT IDENTIFIER",
"oid": "2.23.133.2.2",
"components": [
2,
23,
133,
2,
2
],
"name": "tcg-at-tpmModel"
},
"value": {
"_choice": "utf8String",
"value": "ST33HTPHAHC0"
}
}
],
[
{
"_type": "AttributeTypeAndValue",
"type": {
"_type": "OBJECT IDENTIFIER",
"oid": "2.23.133.2.3",
"components": [
2,
23,
133,
2,
3
],
"name": "tcg-at-tpmVersion"
},
"value": {
"_choice": "utf8String",
"value": "id:00490008"
}
}
]
]
}
}
]
},
{
"_type": "Extension",
"extnID": {
"_type": "OBJECT IDENTIFIER",
"oid": "2.5.29.9",
"components": [
2,
5,
29,
9
],
"name": "id-x509-ce-subjectDirectoryAttributes"
},
"critical": false,
"extnValue": "305E301706056781050210310E300C0C03322E300201000202008A304306056781050212313A30380201000101FFA0030A0101A1030A0100A2030A0100A310300E1603332E310A01040A01020101FFA40F300D16053134302D320A0102010100",
"_extnValue_choice": "ext-SubjectDirectoryAttributes",
"_extnValue": [
{
"_type": "AttributeSet",
"type": {
"_type": "OBJECT IDENTIFIER",
"oid": "2.23.133.2.16",
"components": [
2,
23,
133,
2,
16
],
"name": "tcg-at-tpmSpecification"
},
"values": [
"300C0C03322E300201000202008A"
],
"_values_choice": "at-TPMSpecification",
"_values": [
{
"_type": "TPMSpecification",
"family": "2.0",
"level": 0,
"revision": 138
}
]
},
{
"_type": "AttributeSet",
"type": {
"_type": "OBJECT IDENTIFIER",
"oid": "2.23.133.2.18",
"components": [
2,
23,
133,
2,
18
],
"name": "tcg-at-tpmSecurityAssertions"
},
"values": [
"30380201000101FFA0030A0101A1030A0100A2030A0100A310300E1603332E310A01040A01020101FFA40F300D16053134302D320A0102010100"
],
"_values_choice": "at-TPMSecurityAssertions",
"_values": [
{
"_type": "TPMSecurityAssertions",
"version": 0,
"fieldUpgradable": true,
"ekGenerationType": "ekgt-injected",
"ekGenerationLocation": "tpmManufacturer",
"ekCertificateGenerationLocation": "tpmManufacturer",
"ccInfo": {
"_type": "CommonCriteriaMeasures",
"version": "3.1",
"assurancelevel": "ealevel4",
"evaluationStatus": "evaluationCompleted",
"plus": true,
"strengthOfFunction": null,
"profileOid": null,
"profileUri": null,
"targetOid": null,
"targetUri": null
},
"fipsLevel": {
"_type": "FIPSLevel",
"version": "140-2",
"level": "sllevel2",
"plus": false
},
"iso9000Certified": false,
"iso9000Uri": null
}
]
}
]
},
{
"_type": "Extension",
"extnID": {
"_type": "OBJECT IDENTIFIER",
"oid": "2.5.29.15",
"components": [
2,
5,
29,
15
],
"name": "id-x509-ce-keyUsage"
},
"critical": true,
"extnValue": "03020520",
"_extnValue_choice": "ext-KeyUsage",
"_extnValue": [
"keyEncipherment"
]
},
{
"_type": "Extension",
"extnID": {
"_type": "OBJECT IDENTIFIER",
"oid": "2.5.29.19",
"components": [
2,
5,
29,
19
],
"name": "id-x509-ce-basicConstraints"
},
"critical": true,
"extnValue": "3000",
"_extnValue_choice": "ext-BasicConstraints",
"_extnValue": {
"_type": "BasicConstraints",
"cA": false,
"pathLenConstraint": null
}
},
{
"_type": "Extension",
"extnID": {
"_type": "OBJECT IDENTIFIER",
"oid": "2.5.29.37",
"components": [
2,
5,
29,
37
],
"name": "id-x509-ce-extKeyUsage"
},
"critical": false,
"extnValue": "300706056781050801",
"_extnValue_choice": "ext-ExtKeyUsage",
"_extnValue": [
{
"_type": "OBJECT IDENTIFIER",
"oid": "2.23.133.8.1",
"components": [
2,
23,
133,
8,
1
],
"name": "tcg-kp-EKCertificate"
}
]
},
{
"_type": "Extension",
"extnID": {
"_type": "OBJECT IDENTIFIER",
"oid": "1.3.6.1.5.5.7.1.1",
"components": [
1,
3,
6,
1,
5,
5,
7,
1,
1
],
"name": "id-pkix-pe-authorityInfoAccess"
},
"critical": false,
"extnValue": "303C303A06082B06010505073002862E687474703A2F2F7365637572652E676C6F62616C7369676E2E636F6D2F73746D74706D656B696E7430352E637274",
"_extnValue_choice": "ext-AuthorityInfoAccess",
"_extnValue": [
{
"_type": "AccessDescription",
"accessMethod": {
"_type": "OBJECT IDENTIFIER",
"oid": "1.3.6.1.5.5.7.48.2",
"components": [
1,
3,
6,
1,
5,
5,
7,
48,
2
],
"name": "id-pkix-ad-caIssuers"
},
"accessLocation": {
"_choice": "uniformResourceIdentifier",
"value": "http://secure.globalsign.com/stmtpmekint05.crt"
}
}
]
}
]
},
"signatureAlgorithm": {
"_type": "AlgorithmIdentifier",
"algorithm": {
"_type": "OBJECT IDENTIFIER",
"oid": "1.2.840.113549.1.1.11",
"components": [
1,
2,
840,
113549,
1,
1,
11
],
"name": "id-pkcs1-sha256WithRSAEncryption"
},
"parameters": "0500"
},
"signatureValue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
}
```
(Notice that OID names look a bit weird. For reasons that may have been
lost to time and may no longer be relevant, these OIDs are defined with
slightly different names in the ASN.1 modules in Heimdal's source tree.
We'll fix this eventually.)
- Unconstrained integer types have a large integer representation in C that is
not terribly useful in common cases. Range and member constraints on
integer types cause the compiler to use `int`, `int64_t`, `unsigned int`,
and/or `uint64_t` as appropriate.
- The Heimdal ASN.1 compiler currently handles a large subset of X.680, and
(in a branch) a small subset of X.681, X.682, and X.683, which manifests as
automatic handling of all open types contained in `SET`/`SEQUENCE` types
that are parameterized with information object sets. This allows all open
types in PKIX certificates, for example, to get decoded automatically no
matter how deeply nested. We use a TCG EK certificate that has eight
certificate extensions, including subject alternative names and subject
directory attributes where the attribute values are not string types, and
all of these things get decoded automatically.
- The template backend dedups templates to save space. This is an O(N^2) kind
of feature that we need to make optional, but it works. (When we implement
JER this will have the side-effect of printing the wrong type names in some
cases because two or more types have the same templates and get deduped.)
- There is an _experimental_ ASN.1 module -> JSON feature in the compiler. It
currently dumps type and value definitions, but not class, or object set
definitions. Even for types, it is not complete, and the JSON schema used
is subject to change *WITHOUT NOTICE*.
Perhaps eventually we can re-write the compiler as a C-coded ASN.1 -> JSON
stage followed by a jq-coded code and template generator state, which would
make it much easier to extend the compiler.
...
## Limitations
- `libasn1`'s and, therefore, `asn1_print`'s JSON support is not X.697 (JER)
compatible.
- Control over C types generated is very limited, mainly only for integer
types.
- When using the template backend, `SET { .. }` types are currently not sorted
by tag as they should be, but if the module author sorts them by hand then
DER will be produced.
- `REAL` is not supported.
- `EmbeddedPDV` is not supported.
- `BMPString` is not supported.
- IA5String is not properly supported -- it's essentially treated as a
`UTF8String` with a different tag. This is true of all the string types.
- Only types can be imported at this time. Without some rototilling we likely
will not be able to import anything other than types, values, and object
sets.
- Only simple value syntax is supported. Constructed value syntax (i.e.,
values of `SET`, `SEQUENCE`, `SET OF`, and `SEQUENCE OF`), is not supported.
Values of `CHOICE` types are also not supported.
- There is no way to substitute object sets at run-time. This means that
automatic decoding through open types will spend more CPU cycles than the
application might want, by decoding more types than the application might
care about. The ability to substitute object sets at run-time would require
a change to the APIs generated.
- ...
## Compiler Usage
The various options for the Heimdal ASN.1 compiler are described in its manual
page, which is included below.
The `--option-file=FILE` option is particularly useful, as it allows additional
compiler options to be read from a file.
The `--preserve-binary=TYPE-NAME` option is critical for signature validation
as it causes the decoder to save the encoding of the given type so that
signature validation code can easily find the original encoding and thus avoid
having to re-encode or resort to other hacks. E.g., we use this for preserving
the original encoding of the `tbsCertificate` field of `Certificate`.
The `--sequence=TYPE-NAME` causes the compiler to generate additional utility
functions for adding or removing items from the named type when it is a
`SEQUENCE OF` or `SET OF` type.
See the manual page `asn1_compile.1`:
```text
ASN1_COMPILE(1) BSD General Commands Manual ASN1_COMPILE(1)
NAME
asn1_compile — compile ASN.1 modules
SYNOPSIS
asn1_compile [--template] [--prefix-enum] [--enum-prefix=PREFIX]
[--encode-rfc1510-bit-string] [--decode-dce-ber]
[--support-ber] [--preserve-binary=TYPE] [--sequence=TYPE]
[--decorate=DECORATION] [--one-code-file] [--gen-name=NAME]
[--option-file=FILE] [--original-order] [--no-parse-units]
[--type-file=C-HEADER-FILE] [--version] [--help]
[FILE.asn1 [NAME]]
DESCRIPTION
asn1_compile compiles an ASN.1 module into C source code and header
files.
A fairly large subset of ASN.1 as specified in X.680, and the ASN.1 In-
formation Object System as specified in X.681, X.682, and X.683 is sup-
ported, with support for the Distinguished Encoding Rules (DER), partial
Basic Encoding Rules (BER) support, and experimental JSON support (encod-
ing only at this time).
See the compiler's README files for details about the C code and inter-
faces it generates.
The Information Object System support includes automatic codec support
for encoding and decoding through “open types” which are also known as
“typed holes”. See RFC 5912 for examples of how to use the ASN.1 Infor-
mation Object System via X.681/X.682/X.683 annotations. See the com-
piler's README files for more information on ASN.1 Information Object
System support.
Extensions specific to Heimdal are generally not syntactic in nature but
rather command-line options to this program. For example, one can use
command-line options to:
• enable decoding of BER-encoded values;
• enable RFC1510-style handling of ‘BIT STRING’ types;
• enable saving of as-received encodings of specific types
for the purpose of signature validation;
• generate add/remove utility functions for array types;
• decorate generated ‘struct’ types with fields that are nei-
ther encoded nor decoded;
etc.
ASN.1 x.680 features supported:
• most primitive types (except BMPString and REAL);
• all constructed types, including SET and SET OF;
• explicit and implicit tagging.
Size and range constraints on the ‘INTEGER’ type cause the compiler to
generate appropriate C types such as ‘int’, ‘unsigned int’, ‘int64_t’,
‘uint64_t’. Unconstrained ‘INTEGER’ is treated as ‘heim_integer’, which
represents an integer of arbitrary size.
Caveats and ASN.1 x.680 features not supported:
• JSON encoding support is not quite X.697 (JER) compatible.
Its JSON schema is subject to change without notice.
• Control over C types generated is very limited, mainly only
for integer types.
• When using the template backend, `SET { .. }` types are
currently not sorted by tag as they should be, but if the
module author sorts them by hand then correct DER will be
produced.
• ‘AUTOMATIC TAGS’ is not supported.
• The REAL type is not supported.
• The EmbeddedPDV type is not supported.
• The BMPString type is not supported.
• The IA5String is not properly supported, as it's essen-
tially treated as a UTF8String with a different tag.
• All supported non-octet strings are treated as like the
UTF8String type.
• Only types can be imported into ASN.1 modules at this time.
• Only simple value syntax is supported. Constructed value
syntax (i.e., values of SET, SEQUENCE, SET OF, and SEQUENCE
OF types), is not supported. Values of `CHOICE` types are
also not supported.
Options supported:
--template
Use the “template” backend instead of the “codegen” backend
(which is the default backend).
The template backend generates “templates” which are akin to
bytecode, and which are interpreted at run-time.
The codegen backend generates C code for all functions directly,
with no template interpretation.
The template backend scales better than the codegen backend be-
cause as we add support for more encoding rules and more opera-
tions (we may add value comparators) the templates stay mostly
the same, thus scaling linearly with size of module. Whereas the
codegen backend scales linear with the product of module size and
number of encoding rules supported.
--prefix-enum
This option should be removed because ENUMERATED types should al-
ways have their labels prefixed.
--enum-prefix=PREFIX
This option should be removed because ENUMERATED types should al-
ways have their labels prefixed.
--encode-rfc1510-bit-string
Use RFC1510, non-standard handling of “BIT STRING” types.
--decode-dce-ber
--support-ber
--preserve-binary=TYPE
Generate a field named ‘_save’ in the C struct generated for the
named TYPE. This field is used to preserve the original encoding
of the value of the TYPE.
This is useful for cryptographic applications so that they can
check signatures of encoded values as-received without having to
re-encode those values.
For example, the TBSCertificate type should have values preserved
so that Certificate validation can check the signatureValue over
the tbsCertificate's value as-received.
The alternative of encoding a value to check a signature of it is
brittle. For types where non-canonical encodings (such as BER)
are allowed, this alternative is bound to fail. Thus the point
of this option.
--sequence=TYPE
Generate add/remove functions for the named ASN.1 TYPE which must
be a ‘SET OF’ or ‘SEQUENCE OF’ type.
--decorate=ASN1-TYPE:FIELD-ASN1-TYPE:fname[?]
Add to the C struct generated for the given ASN.1 SET or SEQUENCE
type named ASN1-TYPE a “hidden” field named fname of the given
ASN.1 type FIELD-ASN1-TYPE, but do not encode or decode it. If
the fname ends in a question mark, then treat the field as OP-
TIONAL.
This is useful for adding fields to existing types that can be
used for internal bookkeeping but which do not affect interoper-
ability because they are neither encoded nor decoded. For exam-
ple, one might decorate a request type with state needed during
processing of the request.
--decorate=ASN1-TYPE:heim_object_t:fname
Add to the C struct generated for the given ASN.1 SET or SEQUENCE
type named ASN1-TYPE a “hidden” field named fname of C type
‘heim_object_t’ values of which will be copied and released with
‘heim_release()’ and ‘heim_retain()’ respectively.
--decorate=ASN1-TYPE:void*:fname
Add to the C struct generated for the given ASN.1 SET or SEQUENCE
type named ASN1-TYPE a “hidden” field named fname of type ‘void
*’ (but do not encode or decode it.
The destructor and copy constructor functions generated by this
compiler for ASN1-TYPE will set this field to the ‘NULL’ pointer.
--decorate=ASN1-TYPE:FIELD-C-TYPE:fname[?]:[copyfn]:[freefn]:header
Add to the C struct generated for the given ASN.1 SET or SEQUENCE
type named ASN1-TYPE a “hidden” field named fname of the given
external C type FIELD-C-TYPE, declared in the given header but do
not encode or decode this field. If the fname ends in a question
mark, then treat the field as OPTIONAL.
The header must include double quotes or angle brackets. The
copyfn must be the name of a copy constructor function that takes
a pointer to a source value of the type, and a pointer to a des-
tination value of the type, in that order, and which returns zero
on success or else a system error code on failure. The freefn
must be the name of a destructor function that takes a pointer to
a value of the type and which releases resources referenced by
that value, but does not free the value itself (the run-time al-
locates this value as needed from the C heap). The freefn should
also reset the value to a pristine state (such as all zeros).
If the copyfn and freefn are empty strings, then the decoration
field will neither be copied nor freed by the functions generated
for the TYPE.
NOTE: At this time only one decoration may be specified per type.
--one-code-file
Generate a single source code file. Otherwise a separate code
file will be generated for every type.
--gen-name=NAME
Use NAME to form the names of the files generated.
--option-file=FILE
Take additional command-line options from FILE.
--original-order
Attempt to preserve the original order of type definition in the
ASN.1 module. By default the compiler generates types in a topo-
logical sort order.
--no-parse-units
Do not generate to-int / from-int functions for enumeration
types.
--type-file=C-HEADER-FILE
Generate an include of the named header file that might be needed
for common type defintions.
--version
--help
NOTES
Currently only the template backend supports automatic encoding and de-
coding of open types via the ASN.1 Information Object System and
X.681/X.682/X.683 annotations.
HEIMDAL February 22, 2021 HEIMDAL
```
## APIs Generated by the Compiler
Every named type in an ASN.1 module gets a corresponding type in C.
Em-dashes in symbols become underscores.
Every named type in an ASN.1 module also gets several functions generated
associated with it:
- `int decode_TypeName(const unsigned char *, size_t, TypeName *, size_t *);`
Decodes a value of `TypeName` in the given byte array of the given size,
into the given `TypeName` object, and outputs the number of bytes parsed.
Returns 0 on success, or an error that can be formatted as a string using
the `com_err` library.
- `int encode_TypeName(unsigned char *, size_t, const TypeName *, size_t *);`
Encodes the given object of `TypeName` type into the given byte array of the
given size, outputting the number of bytes used.
NOTE WELL: the `unsigned char *` pointer must point to the _last_ byte of
the buffer!
Returns 0 on success, or an error that can be formatted as a string using
the `com_err` library, including system errors such as `ENOMEM`.
- `int length_TypeName(const TypeName *);`
Returns the number of bytes needed to encode the given object.
- `void free_TypeName(TypeName *);`
Releases the memory associated with the content of the given object, but
note that the object itself is _not_ released.
- `int copy_TypeName(const TypeName *, TypeName *);`
Copies the content of the given `const` object to the destination,
non-`const` object.
Returns 0 on success, or an error that can be formatted as a string using
the `com_err` library, including system errors such as `ENOMEM`.
- `char *print_TypeName(const TypeName *, int);`
Returns a string (JSON) representation of the given object. The `int`
argument is a bitset of flags:
- `ASN1_PRINT_INDENT`
Indent the JSON. If not given the the JSON will have no interstitial
whitespace, including newlines.
You will want to review the data structures generated. They look like:
```C
typedef struct TBSCertificate TBSCertificate;
typedef struct AlgorithmIdentifier AlgorithmIdentifier;
typedef struct ... ...;
/*
Certificate ::= SEQUENCE {
tbsCertificate TBSCertificate,
signatureAlgorithm AlgorithmIdentifier,
signatureValue BIT STRING {
},
}
*/
typedef struct Certificate {
TBSCertificate tbsCertificate;
AlgorithmIdentifier signatureAlgorithm;
heim_bit_string signatureValue;
} Certificate;
/*
TBSCertificate ::= SEQUENCE {
version [0] Version OPTIONAL,
serialNumber CertificateSerialNumber,
signature AlgorithmIdentifier,
issuer Name,
validity Validity,
subject Name,
subjectPublicKeyInfo SubjectPublicKeyInfo,
issuerUniqueID [1] IMPLICIT BIT STRING {
} OPTIONAL,
subjectUniqueID [2] IMPLICIT BIT STRING {
} OPTIONAL,
extensions [3] Extensions OPTIONAL,
}
*/
typedef struct TBSCertificate {
heim_octet_string _save;
Version *version;
CertificateSerialNumber serialNumber;
AlgorithmIdentifier signature;
Name issuer;
Validity validity;
Name subject;
SubjectPublicKeyInfo subjectPublicKeyInfo;
heim_bit_string *issuerUniqueID;
heim_bit_string *subjectUniqueID;
Extensions *extensions;
} TBSCertificate;
```
Note how trivial the mapping onto C is.
`OPTIONAL` fields become pointer fields, with `NULL` indicating
absence and non-NULL indicating presence.
And so on.
## `asn1_print` Usage
```text
ASN1_PRINT(1) BSD General Commands Manual ASN1_PRINT(1)
NAME
asn1_print — dump ASN.1 DER encoded values
SYNOPSIS
asn1_print [-i | --no-indent] [-I | --inner] [-l | --list-types]
[-A | --try-all-types] [-S | --raw-sequence] [-n | --no-print]
[-q | --quiet] [--test-encode] [--test-copy]
[-l -v | --version] [-l -h | --help] [FILE [TypeName...]]
DESCRIPTION
asn1_print Dumps ASN.1 DER-encoded values. If one or more TypeName argu-
ments are given, then asn1_print will print the value in a JSON-like for-
mat using its knowledge of the ASN.1 modules defining those types, stop-
ping at the first type for which it can successfully decode the value.
If TypeNames are given, they must be the names of ASN.1 types exported by
an ASN.1 modules that are compiled into asn1_print. Use the
--try-all-types option to attempt decoding as all ASN.1 types known to
asn1_print. If neither any TypeName nor --try-all-types are given, then
the value will be parsed and displayed using just the self-describing
nature of DER.
Options supported:
-i, --no-indent
Do not indent dump.
-I, --inner
Try to parse inner structures of OCTET STRING and constructed
values.
-l, --list-types
List all types known to asn1_print.
-A, --try-all-types
Attempt to decode the value as any of all types known to
asn1_print.
-S, --raw-sequence
If a value parses as a given TypeName but any bytes are left
over, try to parse those separately as well until all bytes are
consumed or an error occurs.
-n, --no-print
For the case where -A or --try-all-types or where a TypeName is
given, do not output a JSON representation of the value, just
attempt to decode it. This is useful for fuzzing.
-q, --quiet
Similar to -n, --no-print but JSON output will be formatted, just
not output. As with -n, --no-print, this option requires -A -/
--try-all-types or that a TypeName be given. This is useful for
fuzzing.
--test-encode
Check that encoding produces the same value as decoding. Useful
for fuzzing.
--test-copy
Test copy functions. Useful for fuzzing.
-v, --version
-h, --help
HEIMDAL February 22, 2021 HEIMDAL
```
## Implementation
See:
- `lib/asn1/main.c` for the `main()` function of the compiler
- `lib/asn1/asn1parse.y` for the grammar and most of the parsing into an AST
- `lib/asn1/symbol.h` for the types making up the AST
- `lib/asn1/{hash,symbol}.c` for compiler AST supporting functionality
- `lib/asn1/gen.c` for the C header file generator
- `lib/asn1/gen_template.c` for the template generator
- `lib/asn1/gen_{decode,encode,length,copy,free}.c` for the C code generator
- `lib/asn1/gen_{glue,seq}.c` for misc code generator
- `lib/asn1/template.c` for the template interpreter
- `lib/asn1/der*.c` for primitive type primitives
- `lib/asn1/extra.c` for primitives related to `ANY`
...
## Futures
- Add JER support so we can convert between JER and DER?
- Add XDR support? There are no ASN.1 Encoding Rules based on XDR, but it is
trivial to construct such for at least that subset of ASN.1 for which the
XDR syntax has equivalent semantics.
- Add OER support?
- Add NDR support? There are no ASN.1 Encoding Rules based on NDR, but it is
trivial to construct such for at least that subset of ASN.1 for which the
IDL syntax has equivalent semantics.
- Perhaps third parties will contribute more control over generated types?
This may require separate publication of the Heimdal ASN.1 compiler from the
rest of Heimdal.
## Moving From C
- Generate and output a JSON representation of the compiled ASN.1 module.
- Code codegen/templategen backends in jq or Haskell or whatever.
- Code template interpreters in some host language.
- Eventually rewrite the compiler itself in Rust or whatever.
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